The present invention relates to a method of driving an optical shutter array, an image forming device, etc., using a liquid crystal, and more particularly to a liquid crystal device using a bistable liquid crystal, particularly a ferroelectric liquid crystal and a method for driving the same.
Hitherto, liquid crystal display devices, which are well known, comprise scanning electrodes and data electrodes arranged in a matrix manner, and a liquid crystal compound is filled between the electrodes to form a large number of picture elements thereto to display images or information. These display devices employ a time-sharing or time-division driving method which comprises the steps of selectively applying address signals sequentially and cyclically to the scanning lines, and, in parallel therewith, selectively applying predetermined information signals to the group of signal electrodes in synchronism with the address signals. However, these display devices and the driving method therefor have a serious drawback as will be described below.
Namely, the drawback is that it is difficult to obtain a high density of picture elements or a large image area. Because of relatively high response speed and low power dissipation, among prior art liquid crystals, most of the liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters, Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In the liquid crystals of this type, molecules of nematic liquid crystal which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of the liquid crystal layers (helical structure), and molecules of these liquid crystals are aligned or oriented parallel to each other in the surfaces of both electrodes. On the other hand, nematic liquid crystals which show positive dielectric anisotropy under application of an electric field are oriented or aligned in the direction of the electric field. Thus, they can cause optical modulation. When display devices of a matrix electrode arrangement are constituted using liquid crystals of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to electrode surfaces is applied to regions (selected points) where scanning electrodes and signal electrodes are selected at a time, whereas a voltage is not applied to regions (non-selected points) where scanning electrodes and signal electrodes are not selected and, accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers arranged in a crossnicol relationship, i.e., with their polarizing axes being substantially perpendicular to each other, are arranged on the upper and lower sides of a liquid crystal cell thus formed, light does not transmit at selected points while it transmits at non-selected points. Thus, the liquid crystal cell can function as an image device.
However, when a matrix electrode structure is constituted, a certain electric field is applied to regions where scanning electrodes are selected and signal electrodes are not selected or regions where scanning electrodes are not selected and signal electrodes are selected (which regions are so called "half-selected points"). If the difference between a voltage applied to the selected points and a voltage applied to the half-selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, the display device normally operates. However, in fact, as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point, when a whole image area (corresponding to one frame) is scanned, decreases with a ratio of 1/N. For this reason, the larger the number of scanning lines, the smaller is the voltage difference as an effective value applied to a selected point and non-selected points when scanning is repeatedly effected. As a result, this leads to unavoidable drawbacks of lowering of image contrast or occurrence of crosstalk. These phenomena result in problems that cannot be essentially avoided, which appear when a liquid crystal not having bistability (which shows a stable state where liquid crystal molecules are oriented or aligned in a horizontal direction with respect to electrode surfaces, but are oriented in a vertical direction only when an electric field is effectively applied) is driven, i.e., repeatedly scanned, by making use of time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc., have already been proposed. However, these methods are not sufficient to overcome the above-mentioned drawbacks. As a result, at present the development of large image area or high packaging density in respect to display elements is delayed because of the fact that it is difficult to sufficiently increase the number of scanning lines.